In connecting two electrical power systems by means of a conventional structure which uses only a circuit-breaker, excessive current resulting from a fault continues to flow until the fault is detected and the circuit-breaker operates. In order to resolve this problem, in Japanese Unexamined Patent Publication 9-285012/1997 (Tokukaihei 9-285012), the present Applicant proposed a structure for a system interconnection device capable of suppressing this kind of excessive current.
FIG. 12 is a drawing explaining a structure for connecting a commercial power system and a private generating system using a system interconnection device 1 according to Japanese Unexamined Patent Publication 9-285012/1997 above, which is a typical example of conventional art. The system interconnection device 1 is made up of a single-phase rectifying bridge circuit and a direct current (DC) reactor, and is installed at the point of connection between two systems, and realizes prompt suppression of voltage drop due to parallel off or fault.
In FIG. 12, in the commercial power system, a commercial power line 2 supplying a private establishment is connected to a commercial bus line 4 via a circuit-breaker 3. To the commercial bus line 4 are connected a plurality of supply lines 5 for connection to typical loads. In the private generating system, on the other hand, a private power generator 7 is connected to a private bus line 6 via a circuit-breaker 8. To the private bus line 6 are connected supply lines 9 for connection to important loads amounting to, for example, 60% to 70% of the private generating capacity. The commercial bus line 4 and the private bus line 6 are interconnected by the system interconnection device 1.
The system interconnection device 1 is interposed in a bus interconnection line 10 connecting the respective bus lines 4 and 6, and includes a single-phase rectifying bridge circuit made up of a pair of thyristors th1 and th2 (which are rectifying switching elements) and a pair of diodes d1 and d2, a DC reactor l, and a circuit-breaker 11. The pair of thyristors th1 and th2 are connected to one of two alternating current (AC) terminals ac1 and ac2 in the bus interconnection line 10, and the pair of diodes d1 and d2 are connected to the other of the AC terminals ac1 and ac2 (in the example shown in FIG. 12, the thyristors th1 and th2 are connected to the AC terminal ac1, and the diodes d1 and d2 to the AC terminal ac2). The DC reactor l, on the other hand, is connected between two DC terminals dc1 and dc2.
The foregoing single-phase rectifying bridge circuit may be made up of only diodes, or of only thyristors. However, if there is a thyristor in at least one arm (connected to one of the AC terminals ac1 and ac2), the circuit-breaker 11 may be omitted.
In the system interconnection device 1 structured as above, during normal operation, the gates of the thyristors th1 and th2 are driven so that the thyristors th1 and th2 conduct current, and current flows either in the direction indicated by reference symbol i1 or the direction indicated by reference symbol i2. Accordingly, setting a current attenuation time constant of the DC reactor l to a value not less than 2.5 times the system frequency cycle causes a current i.sub.usu flowing through the DC reactor l to be direct current of a substantially fixed level, and a voltage across the terminals of the DC reactor l is 0 (V.sub.DCL =l(di.sub.usu /dt)=0).
In contrast, when there is, for example, a fault in the commercial bus line 4, as indicated by reference numeral 12, and an excessive current i.sub.fault flows from the private bus line 6 to the commercial bus line 4, as soon as a value of the excessive current i.sub.fault exceeds the level of the direct current i.sub.usu which had been flowing through the DC reactor l up to that point, the current i.sub.fault which attempts to charge the DC reactor l flows into the DC reactor l, and a voltage proportional to a differential value of the excessive current i.sub.fault is produced across the terminals of the DC reactor l (V.sub.DCL =l (di.sub.fault /dt)).
For this reason, a voltage equal to the voltage V.sub.DCL across the terminals of the DC reactor l is produced across the AC terminals ac1 and ac2. In other words, an apparent impedance arises across the AC terminals ac1 and ac2, and a current-limiting effect can be realized which suppresses the current attempting to flow from the private bus line 6 to the commercial bus line 4 through the bus interconnection line 10. While this current-limiting effect operates, the gates of the thyristors th1 and th2 are blocked by an output from a ground or short-circuit protective relay (not shown), or the circuit-breaker 11 is opened, and the private bus line 6 is cut off from the commercial bus line 4 rapidly and with certainty, thus suppressing an instantaneous voltage drop caused by excessive load on the private power generator 7.
The foregoing conventional system interconnection device 1 is highly effective in suppressing instantaneous voltage drop in the bus line which is not faulty, but the rectifying elements account for a disproportionate share of cost and space in the device. In particular, when the insulating level is high, as with special high-voltage and high-voltage systems, it becomes necessary to stack rectifying elements in tower form in each of the four arms of each phase, and not only are the rectifying elements themselves expensive, but structures for insulation are also costly and take up space.
In contrast, Japanese Unexamined Patent Publication 50-59762/1975 (Tokukaisho 50-59762) discloses an AC current-limiting device, shown in FIG. 13, which is interposed serially between two systems, and which realizes a current-limiting effect using a small number of diodes. This conventional AC current-limiting device 20 is made up of two diodes d1 and d2, connected together serially but with inverse polarity, interposed serially in an alternating-current path, and a reactor l1 connected in parallel with the diode d1, and a reactor l2, connected in parallel with the diode d2. Further, to enable the reactors l1 and l2 to realize sufficient current-limiting effect with respect to excessive current due to load fluctuation or fault, their discharge time constant is set high.
However, since the conventional AC current-limiting device 20 is not provided with any structure for cutting the respective systems off from one another, it cannot be used as a system interconnection device. Moreover, if a system interconnection device is structured by replacing the diodes d1 and d2 of the AC current-limiting device 20 with thyristors like those used in the system interconnection device 1 above, further problems arise, in that the two systems are continually connected by the reactors l1 and l2, and excessive voltage is produced by discharge of magnetic field energy by the reactors l1 and l2 when the thyristors are cut off.
Further, if a system interconnection device is structured by serially connecting a circuit-breaker with the AC current-limiting device 20, since, as mentioned above, the discharge time constant is high, an interval from cut-off of the circuit-breaker to re-connection of the two systems must be long enough to allow discharge of the magnetic field energy.